Electrical equipment typically is supplied with electric current values that remain within a fairly narrow range under normal operating conditions. However, momentary or extended current levels may be produced that greatly exceed the levels supplied to the equipment during normal operating conditions. These current variations often are referred to as over-current or fault conditions.
If not protected from over-current or fault conditions, critical and expensive equipment may be damaged or destroyed. Accordingly, it is routine practice for system designers to use a current limiting fuse to protect system components from dangerous over-current or fault conditions.
A current limiting fuse is a protective device that commonly is connected in series with a comparatively expensive piece of electrical equipment so as to protect the equipment and its internal circuitry from damage. When exposed to an over-current condition or fault, the fuse melts or otherwise creates an open circuit. In normal operation, the fuse acts as a conductor of current.
Conventional fuses typically include an elongated outer enclosure or housing made of an electrically insulating material, a pair of electrical terminals at opposite ends of the enclosure for connecting the fuse in series with a conductor, and one or more other electrical components that form a series electrical path between the terminals. These components typically include a fuse element (also called a spider assembly) that will melt or otherwise produce an open circuit upon the occurrence of an over-current or fault situation. The housing of the fuse is constructed so as to withstand the anticipated operating environment and typically is expected to last approximately 20 to 25 years. A filament-wound epoxy tube contains the fuse element and is painted with ultraviolet (UV) inhibiting paint in order to offer UV protection to the tube material, which would otherwise degrade more quickly over time with exposure to a UV source such as sunlight. The fuse element is placed inside the tube and a bonding material such as an epoxy is used to bond the electrical contacts to the inside wall of the fuse tube. Typically, the housing is a prefabricated unit into which the fuse element is inserted. The resulting assembly is then cured during a curing operation in order to harden the epoxy. This method of producing a fuse tends to be expensive because, among other things, special manufacturing techniques are needed for the curing operation. For example, the curing operation requires special equipment and procedures in order to keep the working area clean or else the fuse will not be properly sealed.
Also, centerless grinding of the tube is required in order to produce a uniform surface to receive the electrode. The surface at the end of the tube needs to be uniform and smooth in order to facilitate proper bonding of the tube, the fuse element, and the electrode during the curing operation. The centerless grinding operation tends to be expensive, as is the curing operation and the painting operation using UV resistant paint. Additionally, the pre-formed tube must have a wall with sufficient thickness to provide adequate burst strength and cantilever strength for the fuse. A thicker wall generally results in a higher cost.
An improper seal leads to moisture penetrating the interior of the fuse, which, in turn, leads to early fuse failure. There are two techniques commonly used to seal the ends of the tube. The first technique, described above, uses a curing operation to seal the ends. The second technique, known as magna-forming, uses a magnetic field to crimp the ends. These methods of sealing may lead to problems with leakage and intrusion of moisture into the interior of the fuse.